1. Field of the Invention
This invention relates generally to methods, systems and apparatus for managing digital communications systems. More specifically, the invention relates to dynamically controlling system parameters that affect performance in communication systems such as DSL systems.
2. Description of Related Art
The present invention refers to digital communication systems where the transmission medium typically is copper wiring. Most commonly, the copper wiring consists of twisted pairs (also referred to as “lines” or “loops”) categorized according to several manufacturing specifications (for example, AWG-26, AWG-24, CAT-3, CAT-5, CAT-6). Typical communication systems making use of copper wiring include digital Subscriber Line (DSL) systems, such as ISDN, HDSL, ADSL and VDSL, and Local Area Networks (LAN), such as Ethernet. A transceiver (for example, a user modem) is situated at each end of the communications line that incorporates the copper wiring.
Existing phone lines typically are “bundled” in some way. “Bundling” several pairs (in a binder or otherwise) can improve service to a single user or permit service for multiple users. For example, 1000-BaseT Ethernet utilizes four twisted pairs to achieve a data rate of 250 Mbps per pair, or an aggregate rate of 1 Gbps (shown in FIG. 1). In FIG. 1, a data stream 110 is fed to a first transceiver 120, where the data stream 110 is decomposed into multiple component data streams 130 and, if desired modulated using a modulator 140. The modulated component data stream is transmitted over a twisted pair 150 to a demodulator 160 and re-composed in a second transceiver 170. Data may be sent in the opposite direction by reversing the roles of the various components previously described.
Another application is the use of the telephone loop plant for DSL service, one example of which is shown in FIG. 2. The twisted pairs 210 emanating from each Customer Premises Equipment (CPE) 220 are grouped into one or more binders 230, which converge at a terminus 240 such as a central office (CO), an optical network unit (ONU), or a remote terminal (RT). Of course, hybrid scenarios may also occur, such as the use of multiple pairs by a single DSL customer aiming to improve his overall data rate.
The bundling of twisted pairs arises either out of necessity (for example, the existing telephone loop infrastructure) or because of the benefits of improved performance (for example, 1000-BaseT Ethernet). In either case however, communications in these settings suffer from interference arising from electromagnetic coupling between neighboring pairs, referred to as “crosstalk” interference. This means that any signal received by a modem at the end of a twisted pair generally contains not only the transmitted signal of the specific pair (which itself is likely distorted to some extent), but also distorted signals transmitted on neighboring pairs. It is apparent, therefore, that the transmission characteristics of a specific pair (for example, the pair's transmitted power) can materially influence communication on a neighboring pair due to the induced crosstalk. Therefore, transmissions on neighboring pairs (especially those belonging to a bundle or sharing the same binder) are coupled in certain ways. The interfering signals are commonly treated as noise. However, crosstalk can be identified in some situations. (See U.S. Ser. No. 09/788,267, which is incorporated herein by reference.) If crosstalk coupling functions can be identified, it may be possible to remove the crosstalk interference.
“Unbundling” involves the incumbent local exchange carrier's (ILEC's) lease of a telephone line or some part of its bandwidth to a competitive local exchange carrier (CLEC). Current unbundling practice with DSL service usually allows the CLEC to place modulated signals directly on leased physical copper-pair phone lines, sometimes referred to as the lease of “dark copper.” Such unbundled signals may provide services, and consequently use spectra, that differ among the various service providers. The difference in spectra can aggravate crosstalking incompatibilities caused by electromagnetic leakage between lines existing in close proximity. ILECs and CLECs try to ensure mutual spectral compatibility by standardizing the frequency bands and the power spectral densities that can be used by various DSL services. However, there are many DSL types and bandwidths, and service providers are often competitors, which complicates such spectrum management. Further, the cooperation and connection between spectrum regulators and DSL standards groups is still in early evolution, so that regulators may allow practices different than those presumed in spectrum management.
DSL spectrum management attempts to define the spectra of various DSL services in order to limit the crosstalk between DSLs that may be deployed in the same binder. Such crosstalk can be the limiting factor in determining the data rates and symmetries of offered DSL services at various loop reaches, so spectrum management finds some level of compromise between the various DSL service offerings that may be simultaneously deployed. Spectrum management studies tend to specify some typical and worst-case loop situations, and then proceed to define fixed spectra for each type of DSL to reduce the mutual degradation between services. Such a fixed spectrum assignment may not produce the desired level of compromise in situations different from those presumed in the studies.
These enacted rules place strict limits on transmission parameters, controlling performance degradation due to crosstalk by uniformly limiting all parties' transmissions in the system. Typically, the entire set of rules applies equally irrespective of the actual crosstalk environment (for example, whether neighboring pairs actually transmit signals or not), thereby providing protection for a worst-case scenario.
Currently, communication parameters at the physical layer (such as transmitted power, transmission bandwidth, transmitted power spectral density, energy allocation in time/frequency, bit allocation in time/frequency) are determined based on static information about a pair of modems and their twisted pair line. As seen in FIG. 3, an existing system 300 has modem pairs 310, 311 connected by twisted pair lines 312. Standardized requirements and constraints 314 for each link are provided to communication adaptation modules 315. In some cases measured line and signal characteristics of a line 312 can be fed back to the communication adaptation module 315 by a module 316 for a given line to assist in operation of the modem pairs 310, 311 corresponding to the line 312. As illustrated in FIG. 3, however, there is no communication or transfer of line and/or signal characteristics outside of each link and its respective modem pair. Moreover, no independent entity has knowledge of the operation of more than one modem pair and line or of the various pairs' interactions (for example, crosstalk between lines). Instead, the rules, requirements and constraints applied to lines and modems such as those shown in FIG. 3 are designed to accommodate the worst cases of crosstalk or other interference, irrespective of the actual conditions present in the system during operation.
One of the shortcomings of current multi-user communication systems is power control. In typical communication systems, which are interference-limited, each user's performance depends not only on its own power allocation, but also on the power allocation of all other users. Consequently, the system design generally involves important performance trade-offs among different users. The DSL environment can be considered a multi-user system, which would benefit from an advanced power allocation scheme that maximizes or allows selection from most or all of the achievable data rates for multiple DSL modems in the presence of mutual interference.
As mentioned above, DSL technology provides high speed data services via ordinary telephone copper pairs. The DSL environment is considered a multi-user environment because telephone lines from different users are bundled together on the way from the central office, and different lines in the bundle frequently create crosstalk into each other. Such crosstalk can be the dominant noise source in a loop. However, early DSL systems such as ADSL and HDSL are designed as single-user systems. Although single-user systems are considerably easier to design, an actual multi-user system design can realize much higher data rates than those of single-user system designs.
As the demand for higher data rates increases and communication systems move toward higher frequency bands, where the crosstalk problem is more pronounced, spectral compatibility and power control are central issues. This is especially true for VDSL, where frequencies up to 20 MHz can be used.
Power control in DSL systems differs from power control in wireless systems because, although the DSL environment varies from loop to loop, it does not vary over time. Since fading and mobility are not issues, the assumption of perfect channel knowledge is reasonable. This allows the implementation of sophisticated centralized power control schemes. On the other hand, unlike the wireless situation where flat fading can often be assumed, the DSL loops are severely frequency selective. Thus, any advanced power allocation scheme needs to consider not only the total amount of power allocated for each user, but also the allocation of power in each frequency. In particular, VDSL systems suffer from a near-far problem when two transmitters located at different distances from the central offices both attempt to communicate with the central office. When one transmitter is much closer to the central office than the other, the interference due to the closer transmitter often overwhelms the signal from the farther transmitter.
DSL modems use frequencies above the traditional voice band to carry high-speed data. To combat intersymbol interference in the severely frequency selective telephone channel, DSL transmission uses Discrete Multitone (DMT) modulation, which divides the frequency band into a large number of sub-channels and lets each sub-channel carry a separate data stream. The use of DMT modulation allows implementation of arbitrary power allocation in each frequency tone, allowing spectral shaping.
As shown in FIG. 4, a DSL bundle 410 can consist of a number of subscriber lines 412 bundled together which, due to their close proximity, generate crosstalk. Near-end crosstalk (NEXT) 414 refers to crosstalk created by transmitters located on the same side as the receiver. Far-end crosstalk (FEXT) 416 refers to crosstalk created by transmitters located on the opposite side. NEXT typically is much larger than FEXT. The examples of the present invention presented herein use frequency duplexed systems for illustrative purposes.
Current DSL systems are designed as single-user systems. In addition to a system total power constraint, each user also is subject to a static power spectrum density (PSD) constraint. The power spectrum density constraint limits the worst-case interference level from any modem; thus, each modem can be designed to withstand the worst-case noise. Such a design is conservative in the sense that realistic deployment scenarios often have interference levels much lower than the worst-case noise, and current systems are not designed to take advantage of this fact. In addition, the same power spectrum density constraint is applied to all modems uniformly regardless of their geographic location.
The absence of different power allocations for different users in different locations is problematic because of the near-far problem mentioned before. FIG. 5 illustrates a configuration in which two VDSL loops 510 in the same binder emanate from the central office 512 to a far customer premises 514 and a near customer premises 516. When both transmitters at the CPE-side transmit at the same power spectral density, the FEXT 526 caused by the short line can overwhelm the data signal in the long line due to the difference in line attenuation. The upstream performance of the long line is therefore severely affected by the upstream transmission of the short line. To remedy this spectral compatibility problem between short and long lines, the short lines must reduce their upstream power spectral densities so that they do not cause unfair interference into the long lines. This reduction of upstream transmit power spectral density is known as upstream power back-off. Note that the downstream direction does not suffer from a similar problem because, although all transmitters at the CO-side also transmit at the same power spectral density, the FEXT they cause to each other is identical at any fixed distance from CO. This downstream FEXT level is typically much smaller than the data signals, so it does not pose a serious problem to downstream transmission.
Several upstream power back-off methods have been proposed in VDSL. All current power back-off methods attempt to reduce the interference emission caused by shorter loops by forcing the shorter loop to emulate the behavior of a longer loop. For example, in the constant power back-off method, a constant factor is applied across the frequency in upstream transmission bands, so that at a particular reference frequency the received PSD level from shorter loops is the same as the received PSD level from a longer reference loop.
A generalization of this method is called the reference length method where variable levels of back-off are implemented across the frequency so that the received PSD for a shorter loop is the same as some longer reference loop at all frequencies. However, imposing the same PSD limit for shorter loops across the entire frequency band may be too restrictive since high frequency bands usually have too much attenuation to be useful in long loops. Therefore, short loops should be able to transmit at high frequency bands without worrying about their interference.
This observation leads to the multiple reference length method, which sets a different reference length at each upstream frequency band. All of the methods mentioned above equalize the PSD level of a shorter loop to the PSD level of some longer reference loop. While these methods may be easy to implement in some cases, better performance can be obtained if the interference levels themselves are equalized instead. Examples of such approaches are the equalized-FEXT method, which forces the FEXT emission by shorter loops to be equal to the FEXT from a longer reference loop, and the reference noise method which forces the FEXT emission to equal to a more general reference noise. Although there presently is no consensus on a single method, it is clear that a flexible method such as reference noise that allows spectrum shaping is more likely to provide better performance.
Previously proposed power back-off methods require the power or noise spectrum of the short loops to comply with a reference loop or a reference noise. These approaches are simple to implement because each loop only needs to adjust its power spectrum according to a reference and do not require any knowledge of the network configuration. If, however, loop and coupling characteristics in the network are known to either the loops themselves, or a centralized third party, adaptive adjustment power spectrum levels can be implemented, allowing better system performance.
However, the optimization problem involved is complex as a result of the large number of variables and, due to the non-convex nature of the problem, many local minima exist. Early attempts at solving this problem often resorted to added constraints such as all transmitter power spectrum densities being the same, or all PSDs being in some sense symmetrical. The first attempt at finding the true global optimum is based on quantum annealing to minimize the total energy subject to rate constraints on each user.
As will be appreciated by those skilled in the art, the earlier methods described above had various shortcomings. Some of these methods were simple to implement, but forfeited available performance for the sake of such simplicity. The methods that attempted to realize higher performance levels, on the other hand, were too complex to be practical. A relatively simple method and system that can achieve substantial improvement in system performance would represent an important advancement in the art.
As noted above, DSL systems are rapidly gaining popularity as a broadband access technology capable of reliably delivering high data rates over telephone subscriber lines. The successful deployment of Asymmetric DSL (ADSL) systems has helped reveal the potential of this technology. Current efforts focus on VDSL, which allows the use of bandwidth up to 20 MHz. ADSL can reach downstream rates up to 6 Mbps, while VDSL aims to deliver asymmetric service with downstream rates up to 52 Mbps, and symmetric service with rates up to 13 Mbps. However, DSL communication is still far from reaching its full potential, and the gradual “shortening” of loops presents an opportunity to develop advanced methods that can achieve improved rates and performance.
In advanced DSL service the location of the line termination (LT or “central-office side”), as well as network termination (NT or “customer premises side”), can vary. That is, not all LT modems are in the same physical location. Often the location may be an ONU or cabinet, where placement and attachment of CLEC equipment may be technically difficult if not physically impossible. The difficulty arises because CLEC fiber access to the ONU may be restricted and/or the ONU may not be large enough to accommodate a shelf/rack for each new CLEC. Placement of such CLEC equipment for dark copper is often called “collocation” when it is in the central office. While space and facilitation of such central office collocation for unbundling of the dark copper might be mandated by law in some cases, an ILEC may only provide what is essentially packet unbundling at the LT terminal (that is, service bandwidth leased at a layer 2 or 3 protocol level, not at the physical layer). This represents a change in the architecture presumed in many spectrum studies.
Control of all the physical layer signals by a single service provider allows coordination of the transmitted signals in ways that can be beneficial to performance of DSL service. Packet unbundling, which makes available the digital bandwidth on the twisted pairs, rather than the direct physical layer lease of the line itself, is seen to be a likely step in the evolution of DSL service.
A developing DSL system topology is shown in FIG. 6. Some twisted pairs 616 emanate from the CO 610 and reach out to the customer premises 614. The installation of an ONU 612 (at a point between the CO 610 and one or more CPEs 614) shortens loop lengths 618 so that the reach and performance of DSL service are improved. Typically, the ONU 612 is connected to the CO 610 through a fiber link 622. Pairs 616 and 618 can occupy the same binder 620.
Crosstalk coupling is strongest among the twisted-pairs in a binder group. Therefore, eliminating or mitigating self-FEXT within a binder group has the biggest performance benefit. “Unbundled” lines of different service providers may share a binder group which can result in the absence of collocation of the CO transceiver equipment. However, there are indications that ONU deployment will lead to an architecture in which some type of vectored transmission will be necessary since different service providers may have to “share” a fiber link to an ONU (for example, link 622 of FIG. 6) from which individual user lines will emanate and to which they will converge. More specifically, the current architecture of “line unbundling” becomes impractical with the installation of ONUs, since line unbundling implies that each service provider uses its own individual fiber to provide a proprietary connection to the ONU, and that the ONU must be large enough to accommodate a shelf or rack for each service provider. Often, this is not practical or possible. These difficulties may lead to the evolution of “packet unbundling” where service bandwidth is leased at the transport layer, instead of the physical layer. In that case, vectored transmission becomes more appealing because it can offer substantial performance improvement and enhanced control.
The crosstalk problem has been addressed before with some shortcomings. For example, in some systems, MIMO Minimum-Mean-Square-Error (MMSE) linear equalizers were derived. Another prior method employs the singular value decomposition to achieve crosstalk cancellation assuming co-location of both transmitters and receivers. Other earlier methods include “wider than Nyquist” transmitters which were shown to provide performance advantages compared to “Nyquist-limited” ones, and crosstalk cyclostationarity (induced by transmitter synchronization) combined with oversampling which were shown to result in higher SNR values.
None of the earlier methods or systems provided a relatively simple and effective reduction in crosstalk interference in wireline communication systems. However, vectored transmission (as defined in this invention) can achieve a high degree of crosstalk reduction without unreasonable complexity. Moreover, the use of vectored transmission can accommodate the approaching architectural changes coming to DSL service as well as provide an opportunity for dynamic system management which can overcome the shortcomings of prior systems and methods.